A formal Fe(III/V) redox couple in an intercalation electrode

Iron is the most abundant transition metal in Earth’s crust, and redox cycling between its well-known low-valent oxidation states of FeII and FeIII drives crucial processes in nature. The FeII/III redox couple charge compensates cycling of lithium iron phosphate, a positive electrode (cathode) for lithium-ion batteries. High-valent iron redox couples, involving formal oxidation higher than FeIII, could deliver higher electrochemical potentials and energy densities. However, because of the instability of high-valent Fe electrodes, they have proven difficult to probe and exploit in intercalation systems. In this work, we report and characterize a formal FeIII/V redox couple by revisiting the charge compensation mechanism of (de)lithiation in Li4FeSbO6 (LFSO). Valence-sensitive experimental and computational core-level spectroscopy reveal a direct transition from FeIII (3d5) to a negative charge-transfer FeV (3d5L2) ground state upon delithiation, without forming FeIV. Exhibiting resistance to calendar aging, high operating potential, and low voltage hysteresis, the FeIII/V redox couple in LFSO provides a framework for developing sustainable, Fe-based intercalation cathodes for high-voltage applications.